Backups of the GoodNotes library are exported as a zip file and contain documents in .goodnotes format. You can unzip the file and import one or more of its documents into GoodNotes for further editing, or restore your library from the zipped file as a whole.
If you'd like to be able to view your backed-up notes, you can let the app back up notes automatically in PDF format: Just tap the Gear icon in the top-right corner of the Library view > Settings > Automatic Backup > File Format > PDF.